This invention relates to a method for automatically measuring the amount of water in a natural gas stream through the use of a gas chromatograph which is controlled and operated by a computer control unit.
The prior art is replete with devices and methods for the detection and measurement of water, none of which is applicable to the problem of detection and measurement of low water levels in gaseous hydrocarbons flowing in a pipeline. One such device is an infrared absorption type hygrometer. This device takes advantage of the fact that water absorbs electromagnetic radiation in the infrared region, specifically radiation of 1.40 to 1.93 microns in wave length. By measuring the attenuation or decrease in light intensity of a beam of this wave length as it passes through a gas, the moisture content of the sample can be determined. This device is not sensitive enough to detect minute quantities of water and is subject to error caused by interference from any other compounds that absorb radiation in the same wave lenght range. These factors make it unsuitable for use in the present application.
Another type of water measurement device is a conductivity cell. This device is an acid conductivity cell wherein the electrical conductivity of an insulating material coated with sulfuric acid is measured in the environment of varying moisture content. In theory, the conductivity of the cell has a direct correspondence to the water level, and the acid coating on the insulating material becomes more conductive when more water molecules are present on its surface. This device gives a large variation of moisture level readings with changes in ambient temperature. Also, when subjected to either a dry gas flow or to a liquid hydrocarbon, the conductivity of the cell decays rapidly.
Another moisture measurement device is an electrolytic hygrometer. This device employs a hygroscopic salt, such as phosphorous pentoxide, to absorb moisture from a fluid sample and subsequently to perform an electrolysis of water into hydrogen and oxygen. The electrolysis current required is a measure of the amount of moisture present. There are a number of materials that will create problems if they are present in the fluid sample. A phenomenon called the "recombination effect" introduces large errors at low moisture levels in hydrogen-rich or oxygen-rich samples. This instrument is also unstable in the presence of unsaturated monomers, alcohols, amines, ammonia and hydrogen fluoride. Alcohols are seen by the cell as water, and the amines and ammonia react with the desiccant which is used to absorb the moisture.
Still another commercial measurement device is known as the piezoelectric hygrometer. It utilizes twin piezoelectric crystals coated with a hygroscopic material. Water from the fluid sample is absorbed by the crystal coating, increasing the total mass and decreasing the oscillating frequency of the crystal. The moisture level is determined by comparing the frequency of the crystal exposed to the sample gas with one exposed to bone-dry gas. Any liquid present in the sample may be forced into the crystal cell, thereby immediately stopping crystal oscillation. Water will ruin the crystal coating instantly while other solvents may soften the crystal coating and ruin the calibration of the instrument. In addition, such monomers as butadiene and styrene may polymerize and coat the crystals, thus preventing proper operation of the unit. This unit is commonly used to monitor water concentration in natural gas, but because of these problems, it gives unreliable measurements.
Yet another commercially used device is the impedence-type hygrometer, an instrument capable of measuring a wide range of moisture levels. This instrument measure water content of a sample by means of a probe whose electrical impedance is a function of the vapor pressure of the moisture in the fluid. A typical sensor is constructed of porous aluminum oxide. Since the pore wall openings are small in relation to organic molecules, admission into the pore cavity is limited to small molecules such as water. The aluminum oxide sensor is sensitive to water vapor pressure and is, therefore, not affected by large concentrations of petroleum gases, freons, hydrogen sulfide, ozone or sulfur dioxide. Unfortunately, this sensor is affected by polar substances other than water, such as ammonia, methyl amines, and alcohols. The common presence of these materials in pipeline hydrocarbons renders the impedence-type hygrometer unusable in the present application.
The mirror dew point apparatus is still another type of device which has been used to measure the dew point of gas streams. The apparatus consists of a pressure cell containing a highly polished mirror which can be cooled by some common refrigerant, such as propane. A gas stream is passed over the mirror at a convenient pressure, and the temperature of the mirror is slowly reduced until visible condensation occurs. The water content of the gas stream can be determined from established tables by observing the temperature and pressure. This method is listed in the 1969 Book of ASTM Standards, Part 19. It is noted in this text that some gaseous fuels contain vapors of hydrocarbons or other components that easily condense into liquid and sometimes interfere with or mask the water dew point. This method, although it is an ASTM standard, can give questionable results if the dew point of the condensable hydrocarbons is higher than that of the water vapor and those hydrocarbons that are present in large amounts.
If that is the case, these hydrocarbons may flood the mirror and obscure or wash off the water dew point. This condition is especially true for propane and heavier hydrocarbons when water vapor content is quite low, as is the case in natural gas pipelines.
One method that works quite well is the Bureau of Standards Gravimetric Procedure. This method is the primary standard for calibration of all types of gas or moisture analyzers. It employs two water-absorbant materials, magnesium perchlorate and phosphorus pentoxide, to absorb all of the moisture in a specific volume of gas samples. The drying tubes containing the absorbant materials or desiccants are very carefully weighed before and after the gas sample is passed through them. The weight gain is the water which has been absorbed from the sample. This method is extremely accurate, but there are several significant measurement technique problems which render this method unusable for measuring water content at various locations along the length of a pipeline. The handling of the desiccant tubes requires the utmost of care since even fingerprints on the tube's surface will introduce error into the measurements. The desiccants will also absorb other impurities which may be present in the gas sample, thus making this method even more subject to error if it is used at moisture levels in actual pipelines.
Gas chromatographs have long been used to analyze various components in a natural gas stream. However, none of the detectors used had adequate sensitivity for measuring low water levels.
The use of an electron capture detector in connection with a chromatographic column, and tubing made of nonwater-absorbing materials overcomes all of the disadvantages of the prior art devices discussed above. The method of the present invention allows accurate detection and measurement of water at levels well below five parts per million. The development of a highly accurate electron capture device is relatively new. The use of the electron capture detector in gas chromatography is widely used in the analysis of trace quantities of chlorinated hydrocarbons. Highly chlorinated pesticides such as lindane (hexachlorocyclohexane) are detectable at the one picogram level. The electron capture detector is extremely sensitive to water. Response to natural gas hydrocarbons is very low. Some contaminates such as hydrogen sulfide, carbonyl sulfide, and other sulfur compounds are detectable. However, with the proper selection of operating conditions and columns, they do not interfere with water measurement.